Today, we chase a mini-eclipse. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.

Every century or so, Venus
makes two passes between us and the Sun -- about
ten years apart. Unlike our Moon, which blots out
the Sun, Venus is far away. Watch it through a dark
glass and you see only a small black spot moving
across the sun's disc. Venus blots out nothing, but
that crossing, or transit, of the Sun gives the
information we need to calculate the size of the
solar system. Eighteenth-century astronomers saw a
huge scientific payoff in observing and measuring
their two transits.

Now think about solar eclipses -- those infrequent
transits of the Moon across the face of the Sun.
People will travel far to see one. Transits of
Venus are even rarer, and observing the two that
occurred in the 18th century was terribly important
to astronomers.

So a young French astronomer named Jean Chappe set
out to measure the 1761 transit of Venus, which
would be visible in June from Tobolsk in central
Siberia. Chappe left Paris with an expedition to
Tobolsk in November the year before. Astronomy
professor Donald Fernie describes the trip: The
first eight days on the wretched roads from Paris
to Strasburg left all the glass in Chappe's
instruments smashed and his carriages ruined.
Resupplied, he caught a riverboat down the Danube
from Ulm to Vienna. From there, cross-country to
Warsaw where he arrived on January 22nd.

In Warsaw, his group switched to horse-drawn
sleighs. They made it to Moscow on March 17th. Now
they had to cross the Ural Mountains while the
weather stayed cold. Tobolsk lay in a vast
marshland that'd be impassable once the snows
melted. Chappe arrived in mid-April. It'd taken him
five months to get there.

He set up his small observatory on a nearby
mountaintop while spring flooding savaged the
region that year. He was threatened by locals who
thought they were being punished for letting a
foreigner mess with the Sun. But the day came, and
Chappe made measurements that served astronomers
for the next century. It had, in fact, been well
worth the trouble. But trouble it had been.

The second transit that century occurred in 1769.
Chappe died of disease on his journey to Baja
California to measure that one. Captain Cook's
astronomer viewed it from Tahiti. And another
French astronomer went all the way to the
Philippines in a nightmarish four-year voyage, only
to be foiled by cloud cover.

In all, 18th-century astronomers made eight
observations which fixed the distance from Earth to
the sun until Venus made two more transits in 1874
and 1882. We now wait for the next transit on June
7th, 2004. There'll be no wild horse-drawn journeys
across the tundra this time. We'll view this one
from a more remote place than Tobolsk. For now our
telescopes ride in space. No more dreams dashed by
random cloud cover. No more wolves or shipwrecks.
But there will be new adventures, new dangers and
-- new surprises.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.